This invention relates to an optical head and an apparatus having the optical head used for reproducing or recording data from or into an optical information recording medium formed of a plurality of information layers.
In recent years, optical disks such as HD DVD and Blu-ray using the blue semiconductor laser have been manufactured and commercialized as optical information recording media. Especially, Blu-ray realizes a capacity as large as 25 GB per information layer. A further increase in capacity per information layer, however, is not an easy matter due to the limit caused by diffraction of light. The future trend, therefore, is toward realization of large capacity by a plurality of information layers.
As a technique related to this invention, the recording and reproduction technique for an information recording medium having a plurality of information layers is disclosed in JP-A-08-185640 (Paragraph 8, FIG. 1) and JP-A-2006-344344 (Paragraph 29, FIGS. 29 and 33).